Glass shader too dark
When the Glass Shader is applied to a whole, solid object like that, it will treat the entire object as very thick, solid cube of pure glass.

Although it looks odd, what you are seeing is physically correct, if you had a big thick piece of glass like that and a dark gray environment a gray plane as the only source of reflections. It looks odd because you never see glass like that in the real world.

If you add a solidify modifier to your cube, it might look more like what you were expecting. This will make the glass look like a hollow cube (like a fish tank).

If you add more lights or an environment texture, it will start to look more realistic still.